There is significant and increasing interest in utilizing direct methanol fuel cells (DMFCs) for portable power applications. Currently, most attention is focused on DMFC sub-watt power systems that are relatively simple. However, more sophisticated systems, providing more than several watts of power, can provide higher power densities. Stack designs for such systems mirror conventional hydrogen fuel cell approaches, i.e., multiple bipolar plates aligned in series with internal manifolds for reactant delivery and removal.
Over the course of the past four years (1999-2003), great strides have been made in the design of Los Alamos National Laboratory (LANL) DMFC stacks and stack components. These in-house designs have exhibited improved performance, as well as an increase in the stack power densities (based on maximum stack power output). Note that the DMFC-60 and 22-cell Palm Power designs are discussed for illustrative purposes only in order to denote the evolving design work at LANL that has lead to the present invention. The designs of the DMFC-60 and 22-cell Palm Power were not published, nor commercially pursued, as such the DMFC-60 and 22-cell Palm Power are not considered prior art.
The first of the stack designs, the DMFC-60, and specifically its FY00 iteration, included metal flow-fields, rectangular bipolar plates, and rectangular endplates. The performance by this design is detailed in FIG. 1, but specific power (W/kg), as detailed in FIG. 2, suffered due to the use of relatively heavy metal hardware.
The second of the stack designs, the 22-cell Palm Power stack, made use of the same basic membrane-electrode assembly (MEA) and GDL (gas diffusion layer) technology as the DMFC-60, but utilized graphite-based bipolar plates (with integrated flow-fields) as well as composite endplates, contributing to a greatly reduced overall stack weight. Performance for the 22-cell Palm Power stack suffered as a result of cell-to-cell variations caused in part from non-optimized flow-field and GDL combinations. However, power density nearly doubled over the DMFC-60 design (see FIG. 2) because of reduced component mass.
The 12-cell stack design of the present invention provides a substantial increase in average cell current density over the 22-cell design (see FIG. 1). This increase is attributed to thinning the bipolar plates, changing the GDL from the cloth-type used for the previous two stack designs to carbon paper type, optimizing the flow-field designs to reduce cell-to-cell variations, increasing the target operating temperature from 70° C. to 75° C., changing the endplate material to a robust carbon composite, and employing relatively heavy gold-coated stainless steel current collectors on each end of the stack. Note that even though the number of cells in the 12-cell stack design was reduced from the 22-cell design, components such as metal current collectors added inactive mass; thus, the overall power density remained constant at about 80 W/kg.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.